KUALA LUMPUR, Malaysia — Searchers zeroed in Friday on a remote island chain in the Indian Ocean for a missing jet that may have flown for hours after it vanished.

A Malaysian government official involved in the probe told the Associated Press on Friday that investigators are increasingly certain the aircraft turned back after losing contact and that someone with aviation skills was responsible for the change in course.

The official declined to be identified because he is not authorized to brief the media.

Indian ships and planes expanded their search Friday to areas west of the Andaman and Nicobar islands chain, hundreds of miles from the intended course of Flight MH370, said V.S.R. Murty, an Indian Coast Guard inspector-general.

Several media outlets reported that U.S. officials said the flight sent signals to a satellite for four hours after the aircraft vanished early Saturday, raising the possibility the jet with its 239 people aboard could have flown far from the search areas.

ABC News reported that two U.S. officials said two of the jet's communication systems shut down separately shortly after the craft last communicated its position.

The data reporting system, the officials said, was shut down at 1:07 a.m. as the jet was on course an hour after takeoff from Kuala Lumpur to Beijing. The transponder, which transmits location and altitude, shut down at 1:21 a.m.

The officials could not say whether the mechanisms were shut down deliberately by someone in the cockpit or during a possible electrical failure in which various systems shut down one after the other.

Pointing to the communication systems' separate shutdowns, a U.S. official told the Associated Press that investigators are examining the possibility of "human intervention" in the jet's disappearance, adding it may have been "an act of piracy." The official, who wasn't authorized to talk to the media and spoke on condition of anonymity, said it was possible the jet may have landed somewhere.

While searchers moved ahead with the possibility that the jet flew well to the west of its intended flight path after communication ended, seismologists at a Chinese university reported Friday they had detected a slight "seismic event" on the seafloor between Vietnam and Malaysia at the spot where the jet was around the time it vanished.

A map of the South China Sea, posted on the website of the Seismology and Physics of the Earth's Interior Laboratory at the University of Science and Technology of China, notes the possible location of a seismic tremor on the seafloor around the time of the disappearance of a Malaysian flight March 8.(Photo: Seismology and Physics of the Earth's Interior Lab)

The University of Science and Technology of China's Laboratory of Seismology and Physics of the Earth's Interior said in an online statement that signals from two seismic monitor stations in Malaysia indicated that a slight tremor occurred on the seafloor around 2.55 a.m. Saturday, about 90 miles off the southern tip of Vietnam.

"It was a non-seismic zone; therefore, judging from the time and location of the event, it might be related to the missing MH370 flight," the statement said.

The U.S. Geological Survey (USGS) located a magnitude-2.7 earthquake off the west coast of Sumatra at the time of the "seismic event" noted by the Chinese, said Harley Benz of the USGS National Earthquake Information Center in Denver.

Benz said tremors of that size are a daily occurrence in the region, refuting the Morning Post's claim. Other earthquake experts said such seismic activity is unlikely to be caused by a plane crash.

"It's very unlikely that the aircraft would have hit the undersea floor strong enough to cause a seismic event that we could detect," said Paul Caruso, a geophysicist with USGS.

"The bump from the plane hitting bottom of the ocean would not be noticeable," said earthquake expert John Vidale of the University of Washington.

Malaysia's acting Transport minister, Hishammuddin Hussein, refused to comment Friday on the possibility that the jet had flown for hours off course. He said the investigation team will not address or release information about any of those claims or others until the suggestions have been verified and corroborated.

"I hope within a couple of days to have something conclusive," he told the news conference in Kuala Lumpur.

Much of the early search has focused east of Malaysia in the South China Sea, where the aircraft last communicated with air-traffic base stations about an hour after departing for Beijing.

Two days ago, six Indian navy and coast guard ships, plus reconnaissance planes, began searching eastern parts of the Andaman sea. Friday, they headed west of the Andaman and Nicobar islands near the Bay of Bengal.

There are more than 500 islands in that chain, many of which are richly forested and uninhabited.

Thursday, The Wall Street Journal quoted U.S. investigators as saying they suspected the jet stayed in the air for about four hours after its last confirmed contact, citing data automatically transmitted by the jet's satellite communication link.